Many types of implantable devices incorporate a self-contained transducer for converting magnetic energy from an externally-located magnetic field generator to energy usable by the implanted device. In such a system having an implanted device and an externally-located magnetic field generator for powering the device, sizing and design of the power transfer system is important. In order to properly design the power transfer system while at the same time avoiding overdesign, the distance from the implanted device to the magnetic field generator must be known. However for some types of implanted devices the depth of the implanted device in a recipient's body is variable, and is not known until the time of implantation by a surgeon. One example of such a device is an intracranial pressure monitoring device (ICPM) wherein skull thickness varies considerably between recipients and the device must be located so that it protrudes slightly below the inner surface of the skull and contacts the dura, thereby resulting in a variable distance between the top of the implanted device containing a pick-up coil or transducer and the outer surface of the skull. One conventional technique for accommodating an unknown distance between the magnetic field generator and the implanted device includes increasing the transmission power of the external magnetic field generator. However this increased power can result in heating of the implanted device, the excess heat being potentially hazardous to the recipient. A further technique has been to increase the diameter of the pick-up coil in the implanted device. However, physical size constraints imposed on many implanted devices such as the ICPM are critical; and increasing the diameter of the pick-up coil is undesirable in that it increases the size of the orifice which must be formed in the recipient's skull. The concentrator of the present invention solves the above problems by concentrating magnetic lines of flux from the magnetic generator at the implanted pick-up coil, the concentrator being adapted to accommodate distance variations between the implanted device and the magnetic field generator.